(a) Field of the Invention
The present invention relates to an optical repeater and a signal relay method (or a signal repeating method) thereof, and more particularly, to an optical repeater in a mobile communication system using orthogonal frequency division multiplexing (OFDM) and a signal relay method thereof.
(b) Description of the Related Art
In a mobile communication system, a repeater is installed to provide a service to a service unavailable terminal located in a shadow area or enhance a data transfer rate.
Mobile communication standard technologies such as WIMAX or LTE currently employ orthogonal frequency division multiplexing (OFDM) as a modulation scheme. Thus, a repeater technique which is able to effectively relay or repeat a signal in a mobile communication system using OFDM modulation is required.
A related art optical repeater in a mobile communication system may be classified into an analog type of optical repeater and a digital type of optical repeater. The analog type of optical repeater transmits and receives a radio frequency (RF) signal through an optical line, having a disadvantage in that performance is degraded according to an optical transmission distance, while the digital type of optical repeater digitizes a baseband signal and optically transmits the same, having an advantage in that there is no degradation of performance according to an optical transmission distance.
A mobile communication system using a digital optical repeater includes a general base station, a main hub unit (MHU) which accesses the base station, and a plurality of optical repeaters installed in remote areas to form service coverage up to a terminal. The MHU includes a plurality of optical access points, and a plurality of repeaters access the respective optical access points.
In forward communication, the MHU directly matches an intermediate frequency (IF) signal transmitted from the base station to a coaxial cable, down-converts the matched IF signal into a baseband signal, performs conversion from an analog signal into a digital signal, and then transmits the converted digital signal into a repeater installed in a shadow area. The repeater converts the digital optical signal received from the MHU into the analog signal, up-converts it into the RF signal, and then transmits the up-converted RF signal to a terminal through a repeater antenna. Also, the repeater transmits the digital optical signal received from the MHU to other repeaters connected thereto.
Also, in backward communication, the repeater down-converts an RF signal received from a terminal through the repeater antenna into a baseband signal, and performs conversion from an analog signal to a digital signal. Here, when there is an optical signal transmitted from a different repeater connected thereto, the repeater adds the signal from the different repeater and the converted digital signal and transmits the resultant signal to the MHU or the different repeater connected thereto. Then, the MHU converts the digital optical signal received from the repeater into the analog signal, up-converts it into an IF signal, and then inputs the up-converted signal to a base station.
In the case of forward communication in the mobile communication system using the digital optical repeater, since all the repeaters transmit signals transmitted from a base station to a different repeater connected thereto, the signals may act as interference to the terminals belonging to different repeaters. Also, in the case of backward communication, since a repeater adds a signal from a terminal within its coverage area and a signal transmitted from a different repeater and transmits it as an optical signal, noise is generated according to multi-hop. For these reasons, relay efficiency of the optical repeater is inevitably degraded.